DIPSACUS. 251 



the radiating cells meet, threads of gelatinous matter can be seen 

 to protrude under certain circumstances. No apertures, how- 

 ever, can be seen through which the filaments come, therefore 

 it is thought that they extend directly through the cell-walls. 

 They have been shown to consist of protoplasmic matter with 

 which a certain amount of resinous substance is combined, and 

 at times they contract violently, become thicker, and at last 

 form a small ball on the summit of the gland. The contraction 

 can be produced by many chemical and physical agents, e. g., 

 ammonic carbonate. If a filament under the microscope is 

 treated with a drop of a 2 per cent solution of the carbonate, the 

 folio wino- changes occur : The filament contracts, but almost 



o o 



instantly recovers itself, and is once more protruded ; it does 

 not, however, regain its original form or appearance ; instead of 

 consisting of thin elongated ropes of a highly refracting sub- 

 stance, it is converted into necklace-like masses which strongly 

 resemble the aggregations found in the true insectivorous plants. 



Beal has described somewhat similar hairs on some thistles. 



It is not the province of this volume to discuss the singular 

 relationships which are presented by these groups of insec- 

 tivorous plants. Attention must be directed, however, to the fact 

 that Dionrea and Drosera, with their widely different mechan- 

 isms for the capture of insects, belong to the same natural family ; 

 and that Piuguicula and Utricularia, with methods equally di- 

 verse, are veiy nearly allied plants. Such facts can be explained 

 in part by the theory presented in Volume I. page 328, - - the 

 ^ Theory of Descent." 1 



1 The following list will introduce the student to some of the principal 

 works upon insectivorous plants. As the list is chronologically arranged, it may 

 serve as a brief history of the subject. 



1768. John Ellis : " De Diontea muscipula." A letter to Sir Charles Lin- 

 naeus descriptive of the method by which this fly-trap captures insects. 



1782. Eoth : " Von der Eeizbarkeit des sogenannten Sonnenthaues, Drosera 

 rotimdifolia und longifolia " (Beitrage zur Botanik, Bremen, Th. 1, no. iv, pp. 

 60-76). An account of observations begun in 1779 on the irritability of the 

 glands of sun-dew leaves, showing that they respond to contact with in- 

 sects, but not to a pin or bit of straw. Eoth suggests that the plant may 

 possibly receive some nourishment from the insects. (In Darwin's Botanic 

 Garden, 1780, p. 24, it is stated that Whately had made in England observa- 

 tions similar to those of Eoth. ) 



1791. Bartram : "Travels through North and South Carolina, etc." This 

 book contains a short sketch of the capture of insects by Sarracenia. 



1815. Macbride : " On the power of Sarracenia adunca to entrap insects" 

 (Transactions of the Linnrean Society, xii., 1818, 48-52). 



1829. Burnett, in the Quarterly Journal of Science, Literature, and Arts, 



